In modern cellular wireless receivers the use of a zero-IF (direct conversion) receiver architecture (see FIG. 1) has become popular as a high level of integration can be obtained. As a consequence, a low material or component cost can be obtained. However, one of the most severe limitations on the use of direct conversion techniques is the need to provide an extremely high IIP2.
In FIG. 1 the zero-IF receiver 1 includes a filter 2 coupled to an antenna 3, a low noise amplifier (LNA) 4 that feeds 90 degree phase quadrature down conversion mixers 5A, 5B fed from a local oscillator (LO) 5C. The outputs of mixers 5A, 5B are in-phase (I) and quadrature phase (Q) baseband signals that are applied through low pass filters 6A, 6B and variable gain amplifiers 7A, 7B to analog to digital (A/D converters 8A, 8B, respectively.
In a homodyne receiver, the second-order intermodulation introduces undesirable spectral components at baseband, which degrade the receiver sensitivity. For example, if two strong interferers at frequencies f1 and f2 close to the channel of interest experience even-order distortion, they generate a low-frequency interference signal at the difference frequency f1−f2. This may occur in the low noise amplifier (LNA) or in the mixer.
However, if the LNA and mixer are ac-coupled, the low-frequency beat signal generated in the LNA is filtered out. In addition, the double-balanced down conversion mixer topology suppresses even-order distortion. In an ideal mixer the low-frequency beat present at the mixer RF input is up-converted, but in reality such mixers present a finite feedthrough from the RF input to the IF output, which results in a finite IIP2. In general, it is the down conversion mixer that determines the achievable IIP2 of the receiver.
The majority of the active double-balanced mixers utilized in wireless receivers are based on the Gilbert mixer topology (FIG. 2). In general, both the RF input gm-stage (MB, M1–M2) devices and switching devices (M3–M6) contribute to the mixer nonlinearity, and the mixer IIP2 is set by its second-order nonlinearity and mismatches. In order to improve the mixer IIP2 it is essential to reduce the second-order intermodulation products generated in the mixer, since the device matching cannot be improved beyond a certain limit.
The second-order products generated in the mixer RF input gm-stage can be eliminated by the differential pair RF input stage shown in FIG. 2 (MB, M1–M2). However, as the third-order intercept point (IIP3) of the differential pair is worse than the IIP3 of the common-source (emitter) amplifier at a given value of bias, the common-source (emitter) RF input amplifier is usually preferred to the differential pair. Moreover, as the supply voltage (VDD) scales down with transistor technology, the stacking of four devices in the standard Gilbert cell becomes difficult.
The second-order products generated in the mixer RF input gm-stage can also be eliminated by ac-coupling the RF input stage from the switches, as shown in FIG. 3. Unfortunately, the additional current sources MB1 and MB2 and load resistors RL3 and RL4 increase the mixer NF and add additional parasitic capacitance at the common-node of the switching devices. This parasitic capacitance increases the second-order intermodulation generated in the switching devices.
The conventional attempts to overcome the above mentioned problems includes the use of the fully differential RF input stage (such as the differential pair), the use of ac-coupling the RF input stage from the switches, and the use of dynamic matching techniques.
For example, dynamic matching can be used to increase the IIP2 of the down conversion mixers 5A, 5B. Unfortunately, this technique adds additional complexity to the down conversion process, as two additional multipliers or mixers and LO signals are required. First, one additional mixer is needed at the RF or LO path preceding the main mixing process to mix the desired signal from RF to some (RF+IF) frequency. Then, the main mixer down-converts the desired signal to the IF frequency. Finally, another additional mixer down-converts the desired signal from IF to baseband and up-converts the second-order intermodulation products and 1/f-noise to the IF-frequency. In this technique the undesired mixing products can cause problems, and furthermore the additional mixing stages can raise the thermal noise of the entire down-converter.
The foregoing problems are aggravated when supply voltages are reduced. For example, the supply voltage in a modern sub-micron CMOS technology is very low (on the order of only one volt, i.e., 1.2V) for analog and RF circuits.